1. Field of the Invention
The present invention relates to an electric power plug. More precisely, the present invention is directed to an electric power plug having increased structural resistance against traction forces tending to pull the blades out of the plug body.
2. Prior Art
Electric power plugs of this type which are available on the market are shown in FIGS. 6(A) and 6(B).
The electric power plug shown in FIG. 6(A) is composed of a pair of blades 1, a cord 2 comprising a pair of conductors connected to proximal end portions of the blades 1 respectively, and a plug body 3. Proximal end portions of the blades 1 are embedded in the plug body 3 together with the adjacent portion of the cord 2 so that the distal end portions of the blades project out of the plug body 3. The proximal end portion of each blade 1 is provided with a through-hole 4, which is filled with the resin material forming the plug body 3. The resin material charged in the through-holes 4 increases the strength of the blades 2 to some extent against traction forces tending to extract the blades 2 out of the plug body 3.
The plug shown in FIG. 6(B) is composed of a pair of blades 1, a cord 2 comprising a pair of conductors connected to the blades 1 respectively, a rectangular core 5 made of a firm resin supporting the pair of blades 1 at their intermediate portions, and a plug body 3 of molded resin. The core 5 is half-embedded in the plug body with one of its surfaces exposed to the outside. The proximal end portions of the blades 1, that is, the portion of the blades 1 inward of the core 5 are embedded in the plug body 3. The blades 1 are retained by the plug body 3 and the core 5.
Traditionally, the physical strength of the plugs was examined by the following test.
The blades 1 of the electric power plug are fixed onto hooks to suspend the plug body 3, as shown in FIG. 7. Then, a prescribed load M is applied to the plug body 3 in the downward direction for a prescribed period of time T. The blades 1 are required to have enough strength to resist this loading without being extruded from the plug body 3.
As to the conventional power plug shown in FIG. 6(A), the blades 1 are retained against the above traction force by virtue of the engagement with the plug body 3 at the through-holes 4 and the friction force acting between the blades 1 and the plug body 3.
As to the conventional power plug shown in FIG. 6(B), the blades 1 are retained against the above traction force mainly by virtue of the engagement and friction force acting between the blades 1 and the plug body 3. Although the core 5 ensures a tight retention of the blades 1 to some extent, it can not and is not intended to ensure a strong retention of the blades 1. The core is intended to improve the appearance of the plug body 3 by exposing the surface of the core 5 rather than to increase the strength.
In recent years however, strength requirements are becoming more stringent. The UL Standard, for example, requires that the displacement of the blades according to above test have to be not larger than 1.6 mm. This is a severe requirement for the above-mentioned conventional power plugs. One possible solution to meet with this requirement may be to increase the hardness and the strength of the material used for the plug body 3. But this solution causes an unexpected inconvenience, that is, the electric cable extending from the plug body 3 becomes liable to be bent in an acute angle at its junction with the plug body 3 due to an abrupt change of the stiffness at that location. This bending may cause a breakage of the conductors.